1. Field of the Invention
The present invention relates to a drying process for increasing the filling power of tobacco material and an apparatus for carrying out said process.
2. Description of the Prior Art
In a technologically sophisticated flow drying of cut tobacco, in which the tobacco material to be dried is dried in a stream of hot drying gas, the aim is to achieve a combination of to some extent contradictory process objectives. The best solution from the process technological point of view is accordingly to obtain an optimum of the relevant desired functions. The different objective functions can be combined in three groups relating to the product and process characteristics. The group of physical product properties includes substantially the objective functions of good tobacco filling power with relatively low cigarette drawing resistance and low degradation, giving stable cigarette ends. The chemical sensorial product properties form the second group, the optimum of which is characterized by high aroma retention, low influence on components and satisfactory smoke flavour. The third group required for optimum procedure is that of a minimum energy consumption and minimum waste gas emissions from the point of view of environmental protection.
The individual objective functions of the three different objective groups are governed substantially by the process parameters set forth in the following table, i.e. the tobacco moisture before and after the drying, the local heat and mass transfer coefficients between the tobacco surface and the surrounding drying gas during the treatment, and the specific heat of the drying gas.
TABLE ______________________________________ Product property Tobacco filling power, Minimum cigarette draw energy resistance, Smoke flavour, consumption, Process cigarette end ingredients, low parameters stability aroma retention emissions ______________________________________ Tobacco mois- as high cut moisture minimum ture prior as possible, prefered; i.e. possib. to drying about 40% for 18-20% for moisture cut lamina, cut lamina on difference related to wet basis wet basis Tobacco mois- as low at least 12% minimum ture after as possible (cigarette possible drying moisture) exit air Local heat and as high as as low as temperature mass transfer possible during possible during coefficient the treatment the treatment Specific heat of as high as minimum water high water the drying gas possible, vapour vapour for example content to content high water avoid steam vapour content distillation ______________________________________
Optimum physical product properties are achieved by a relatively high tobacco moisture content prior to drying, as a guide 40%, wet weight basis, should be regarded as an upper limit in practice; furthermore a relatively low tobacco moisture content after drying, maximum possible local heat and mass exchange coefficients during the treatment and as high as possible a specific heat of the drying gas, which can for example be achieved by a high water vapour content. In contrast, optimum chemical sensorial product properties require that the tobacco moisture before the drying corresponds substantially to the usual cut tobacco moisture of about 18% to 20% on a wet basis, and the tobacco moisture after drying is not less than the usual cigarette moisture, i.e. about 12%, again on a wet basis. The local heat and mass exchange should be kept as low as possible during the drying; likewise, to avoid steam distillation, the water vapour content in the drying gas should also be kept as low as possible. The required process characteristics to minimise environmental pollution, are represented by as low as possible an exit air temperature and as low as possible a difference in moisture between the tobacco material before and after the drying as well as a low water vapour content in the drying gas.
From DE 34 41 649 A1 a process is known for reducing the moisture content of expanded tobacco in which the expanded tobacco is dried in a drier with hot gas at a temperature within a range of about 340.degree. C. to about 510.degree. C. The residence time within one or more series connected driers is so dimensioned that a tobacco product is obtained having a moisture content of about 3% to about 16% with respect to the weight at the drier output. In particular, the temperature of the drying gas is kept constant within the drier at about 510.degree. C.
DE 31 47 846 A1 discloses a process for improving the filling power of tobacco material by expansion of the moist tobacco material by pressure reduction and subsequent drying to processing moisture content. The tobacco material with a tobacco moisture of 15% to 80% is dried to a moisture content of 2% to 16%, in each case with respect to the moist tobacco material. The temperature of the drying gas is between 50.degree. C. and 1000.degree. C. and preferably is above 100.degree. C. An expansion apparatus is arranged upstream of a drying section and either separated from said drying section or connected thereto to form a unit. Due to the extremely short residence time of the tobacco material to be dried in the expansion apparatus the drying within the expansion apparatus itself can be neglected.
A further process for increasing the volume of comminuted tobacco ribs by impregnation with an impregnating agent containing at least water with subsequent heating of the impregnated tobacco rib parts with a gaseous drying gas containing water vapour is known from DE 30 37 885 A1. The drying gas has a temperature of about 105.degree. C. to about 250.degree. C. The tobacco rib parts are transported by means of a pneumatic transport system through an expansion zone and a drying zone and held for at least about 10 seconds in the expansion and drying zone, being dried to an end moisture content of at least 12.5% by weight. The transport velocity of the tobacco rib parts is preferably reduced in the vertical direction in a cross-sectional widening of the drying zone so that only the parts which are dried to a predetermined drying degree are further conveyed.
In a process known from DE 32 46 513 A1 for drying and loosening cut tobacco the tobacco is introduced into a conduit through which a gas flow with steam and air is conducted with a velocity of more than about 30 m/sec at a temperature in the range from about 260.degree. C. to 370.degree. C. The conduit comprises an elongated tube having a first and second section in tandem array, the first section having a smaller cross-sectional area than the second so that when the gas passes through the pressure in said region decreases. The tobacco within said tube is continuously accelerated without however reaching the velocity of the gas stream.
Processes for improving the filling power of tobacco material in the prior art are carried out in some cases in that the tobacco is impregnated with a vaporizable liquid or a liquefied gas, for example water, CO.sub.2, organic solvents, Freon and the like, and said impregnating agent thereafter rapidly vaporized or sublimed. This process has however the disadvantage that although it furnishes an expanded product with increased filling power the tobacco structure generated is not particularly stable. On the contrary, for example in cigarettes with these products a socalled hot collapse is observed, this describing the collapse of the tobacco structure when smoked.
DE-PS 3,130,778 discloses a process for increasing the filling power of tobacco material by a so called shock treatment in which suitably conditioned tobacco material is dried in a stream of hot and rapidly flowing gas within a very short time, that is in less than 1 second. Due to this shock-like treatment the tobacco surface dries within an extremely short time and forms a sort of protective shell for the still moist tobacco interior. Although satisfactory physical product properties can be achieved with this process, the chemo-sensory and economic/ecological aspects are largely ignored.